Agriculture Reference
In-Depth Information
gene) and
trans
-acting (regulatory gene) expres-
sion QTLs (eQTL) for traits such as grain
weight, maturity, and quality traits in wheat.
Furthermore, genetical genomics studies using
the phenotypic tails of a segregating population
resulted in a suite of potential candidate genes
for two important drought-related traits, tran-
spiration effi ciency (Xue et al., 2006) and WSC
concentration (Xue et al., 2008), that were
enriched for likely candidate genes. These genes
included those involved in growth, photosynthe-
sis, drought-response, and carbohydrate metab-
olism, some of which collocated with QTLs for
these traits.
There have been numerous successful attempts
to manipulate expression of functional genes and
genes involved in signal transduction, as a fi rst
step in evaluating their role in plant response to
water stress. Manipulation of functional genes
related to osmolyte metabolism, stress-responsive
proteins, ROS-scavenging proteins—and of other
genes in a range of plant species including Arabi-
dopsis (
Arabidopsis thaliana
), tobacco (
Nicotiana
tabacum
), rice (
Oryza sativa
L.), and wheat (see
Umezawa et al., 2006 and references therein)—
have resulted in enhanced dehydration or
desiccation tolerance, “survivability,” or “drought-
tolerance” in laboratory-based assays. Transcrip-
tion factors (TFs) are also postulated in the
drought response through regulation of expres-
sion of downstream target genes, especially
members of large TF families, such as AP2/ERF
(including DREB/CBF), bZIP (including AREB/
ABF), NAC, MYB, MYC, Cys2His2 zinc-fi nger,
and WRKY (see Umezawa et al., 2006, and refer-
ences therein). Overexpression of TFs of different
TF families has resulted in enhanced levels of sur-
vivability in model plant species (e.g., Arabidop-
sis), as well as a few studies in wheat and tomato
(
Solanum lycopersicum
). These TFs can act to
enhance or repress transcription. Several genes
encoding signaling factors that function in drought
response have also been identifi ed (see Umezawa
et al., 2006, and references therein). Manipulation
of these genes has resulted in enhanced surviv-
ability and frequently tolerance to more than one
stress in laboratory-based assays.
A key issue that arises from these functional
genomics studies is the nature of the phenotypic
evaluation of transgenic plants or the drought
screening protocol used to generate the material
for RNA isolation. Most of the previously men-
tioned studies report evaluation of survival in
model species Arabidopsis, tobacco, and rice
using laboratory-scale drought screening under
fast-developing water stress. It is likely that
water-stress conditions in the fi eld will generate
very different gene expression responses versus
those in laboratory-grown material (Passioura
2006b), and thus the relevance of these gene
expression changes to actual productivity in
the fi eld is unknown. Furthermore, survivability
and/or tolerance of severe tissue desiccation
have not yet been associated with increased
productivity in the fi eld. Therefore considerable
care must be taken with the conclusions drawn
from laboratory-scale experiments prior to valida-
tion in fi eld-grown material. More recently,
functional genomics studies have increasingly
sourced fi eld-grown material for laboratory
studies, or undertaken fi eld evaluation of trans-
genic lines. For example, in Xue et al. (2006,
2008), RNA was isolated from tissue collected
from fi eld-grown material for functional
genomics assays to identify candidate genes for
drought-related traits, WSC and CID.
More recently, fi eld performance of transgenic
crop plants was evaluated; transgenic maize (
Zea
mays
) plants overexpressing a NF-Y TF or an
osmolyte maintaining gene, vacuolar H
+
-translo-
cating inorganic pyrophosphatase (H
+
PPase),
were shown to confer a grain yield advantage in
water-limited fi eld environments (Li et al., 2007;
Nelson et al., 2007), while transgenic wheat
(Bahieldin et al., 2005) and rice (Xiao et al., 2007)
plants overexpressing a stress-responsive, stress-
protective, late embryogenesis abundant (LEA)
protein, were found to increase yield in water-
stressed fi eld environments. While these few
studies are promising, the variable nature of
rainfed, water-limited environments and the
polygenic control of traits with a proven role in
increasing production in water-limited environ-
ments suggests that manipulation of single genes
